1. Field of the Invention
The present invention relates to a magneto-optical recording medium which incorporates a signal recording layer constituted by a thin film made of an alloy of a rare earth element and transition metal and with which magneto-optical recording with respect to the signal recording layer is performed by a magnetic-field modulation method and a manufacturing method therefor.
2. Description of the Related Art
A magneto-optical recording medium has been put into practical use as an erasable recording medium exhibiting a high recording density. The magneto-optical recording medium incorporates a signal recording layer constituted by a thin film made of an alloy of a rare earth element and transition metal which has vertical magnetic anisotropy. Thus, magneto-optical recording is performed so that a required signal is recorded.
The magneto-optical recording operation is performed such that the signal recording layer is irradiated with a laser beam to raise the temperature of each of small regions in the signal recording layer. Moreover, a recording magnetic field of about 200.+-.100 Oe is applied to a region in the vicinity of the region heated owing to irradiation with the laser beam. Thus, the direction of magnetization of only the regions of the signal recording layer which have been heated owing to irradiation with the laser beam is changed to a direction corresponding to the signal, which must be recorded. As a result, the signal is recorded as the direction of the magnetization of the signal recording layer.
The magneto-optical recording method includes a light-intensity modulation method and a magnetic-field modulation method. The light-intensity modulation method is a method of modulating the intensity of the laser beam with which the signal recording layer is irradiated is modulated in response to the signal which must be recorded. The magnetic-field modulation method is a method of modulation the intensity of the recording magnetic field, which must be applied to the signal recording layer, is modulated in response to the signal which must be recorded.
The magnetic-field modulation method, which is one of the magneto-optical recording methods, is required to, at high speed, switches the polarity of the recording magnetic field in response to the signal which must be recorded. When the polarity of the recording magnetic field is switched at high speed, a great magnetic field cannot easily be applied. Therefore, when the magnetic-field modulation method is employed, there is a necessity that sufficient recording can be performed even with a weak magnetic field of about 100 Oe even if the polarity of the recording magnetic field is switched at high speed.
Therefore, when the magnetic-field modulation method is employed, the magneto-optical recording medium must have substantially the same recording characteristics in a range of the magnetic field from about 100 Oe to about 300 Oe. Namely, the magneto-optical recording medium adapted to the magnetic-field modulation method must have stable recording characteristics even if the intensity of the recording magnetic field is somewhat changed.
To meet the above-mentioned requirement, a means has been devised with which the signal recording layer is formed into a laminate structure to improve the characteristics of the signal recording layer. When the signal recording layer is formed into the laminate structure, there arises a problem in that the manufacturing process becomes too complicated.
Therefore, another method has been devised with which the surface of the signal recording layer is oxidized to improve the characteristics of the signal recording layer. When the surface of the signal recording layer is oxidized, the signal recording layer is formed into a substantially two-layer structure. As a result, the characteristics of the signal recording layer can be improved. That is, oxidation of the surface of the signal recording layer enables a signal recording layer having a substantially laminate structure to be obtained without a necessity using plural types of magnetic materials to form the signal recording layer. Hence it follows that the characteristics of the signal recording layer can be improved.
However, oxidation of the signal recording layer cannot always improve the characteristics. The characteristics sometimes deteriorate according to the oxidizing conditions. In particular, excessive oxidation of the signal recording layer excessively deteriorate the recording characteristic of the manufactured magnetic recording medium.